This project has two major objectives. The first deals with the effects of self-produced locomotion experience on emotional, perceptual, and cognitive development in infancy. The second is to begin to study the processes by which these effects are brought about. Previous research using quasi-experimental methods has made it clear that experience locomoting by crawling or by "walker" devices is an antecedent of a broad set of psychological changes. However, it is not clear whether the antecedent locomotor experiences are a cause, or a maturational forecaster, of the subsequent psychological changes. To disambiguate this state of affairs, three experimental studies manipulating locomotor experience will be conducted. One study contrasts the effects of random assignment to walker versus static "exersaucer" conditions; the second compares the effects of random assignment to an actively controlled powered mobility device (PMD), a passively moved yoked control PMD condition, and a no-movement benchmark intervention. A third intervention involves training prelocomotor infants to crawl. Outcome assessments involve wariness of heights, visual proprioception, the two position hiding task ("A-not-B error"), and a test of position constancy. To achieve the second objective, separate investigations of process will be initiated. These will include (I) testing an explanation of the development of wariness of heights, and involves relating responsiveness to peripheral optic flow to such wariness, and (2) clarifying the respective roles of the development of stable seated posture and of crawling on the use of landmarks to find the location of a hidden object.